The present invention relates to structures including one or more materials having high emissivity across a narrow band of wavelengths within a desired energy band. More particularly, the invention is a composite emitter having a layer composed of a thermally sprayed ceramic oxide selective emitter material.
Selective emitters are composed of one or more materials that emit more energy, i.e., are more emissive, at certain wavelengths than at other wavelengths at a given temperature. Different materials have different emissive characteristics. In useful applications, selective emitter materials are chosen such that their emissive characteristics are most closely matched to the energy wavelengths necessary to perform a particular task.
For example, selective emitters are used in thermophotovoltaic (TPV) devices, which convert thermal energy emitted within a certain narrow band of wavelengths into electrical energy. The thermal energy is provided by an external heat source, such as solar radiation, combustion, nuclear decay or the like. The selective emitter absorbs broadband energy from the heat source and in turn emits energy within the narrower band at which the TPV device is capable of converting thermal energy to electricity. Other examples of useful applications for selective emitters include infrared dryers, such as those used by paper manufacturers to dry paper, thermometers for measuring high temperatures and the like.
FIG. 1 illustrates a typical TPV device 10, which comprises a photovoltaic (PV) cell 12, an emitter 14 and a filter 16. PV cell 12 is made of any conventional PV material, such as Ge, GaSb, InGaAs, GaInSbAs or the like, which generally operates at wavelengths of about 1 micron to about 2.1 microns. Emitter 14 is made of a material that absorbs incident energy Ei from a heat source (not shown) that heats the emitter to a temperature of 1000 K or more, and emits energy Ee in a desired band of wavelengths narrower than the energy band of incident energy Ei. Ideally, the only energy emitted from emitter 14 would be that usable by PV cell 12. However, most materials when heated emit radiation over a very broad spectrum, far beyond the narrow band of the PV material of PV cell 12. Even emitters comprising selective emitters generally emit energy outside of band usable by PV cell.
The energy of wavelengths longer than those utilized by PV cell 12 is wasted infrared energy that reduces the efficiency of TPV device 10. One solution for attenuating the band of unusable energy reaching PV cell 12 is to use an infrared filter, such as filter 16, that reflects energy Er, which contains a large portion of the unusable energy, back toward emitter 14. Of the energy that reaches PV cell 12, the energy within the band of wavelengths usable by the PV cell is transformed into electrical energy Et and the remaining portion is waste energy Ew, which must be removed by cooling the PV cell.
It is known that certain ceramic oxides, when heated to an appropriate temperature, are highly emissive within a narrow band in the infrared spectrum that closely matches the usable energy bands of many PV materials and emit nearly zero energy outside this narrow band. These ceramic oxides include a group of rare earth oxides and a group of refractory metal oxides doped with d-series elements. However, it is impractical to form an emitter made entirely of one or more of these materials due to their fragility. Therefore, it is necessary to form a composite emitter comprising at least one material other than the oxide.
One method of forming a composite emitter is to apply the ceramic oxide selective emitter to a sturdy substrate made of a material other than a ceramic oxide. Typically, the selective emitter and substrate materials have thermal expansion coefficients that are different from one another. This mismatch of thermal expansion coefficients in this embodiment is detrimental to the integrity of the composite structure and often leads to a failure of the bond between the selective emitter layer and the substrate.
U.S. Pat. No. 5,879,473 to Sarraf shows a composite emitter comprising a rare earth selective emitter layer and a sturdy metal substrate. To reduce the likelihood of mechanical failure of the bond between the selective emitter layer and the substrate due to the thermal expansion coefficient mismatch, the composite emitter of the Sarraf patent includes a compliant porous metal powder layer located between the selective emitter layer and the substrate. The selective emitter layer and the porous metal powder layer are hot isostatically pressed onto the metal substrate and are each 2 mm to 3 mm thick. In an alternative embodiment, the composite emitter of the Sarraf patent includes only two layers. The mismatch between the thermal expansion coefficients in this embodiment is accommodated by mixing a gold-plated rare earth selective emitter material in powdered form with a metal powder and pressing the mixture onto a metal substrate to form a combination layer approximately 5 mm thick.
The composite emitters of the Sarraf patent, however, have several disadvantages. First, the relatively thick, i.e., on the order of one or more millimeters, selective emitter layer and compliant intermediate layer of the Sarraf emitters, however, are not desirable due to the large temperature differentials that can develop through these layers. Second, the thick selective emitter layers require more material than necessary and desirable. Third, the hot isostatic bonding process requires that the selective emitter layer be unnecessarily thick. Fourth, to alleviate the thermal expansion coefficient mismatch, a third non-selective-emitter material must be added to the composite emitter.
In a first aspect, the present invention is directed to a composite emitter comprising a substrate having a first surface and a second surface laterally spaced apart from the first surface. A thermally sprayed layer confronts the first surface of the substrate and comprises a first selective emitter material. A thermally sprayed layer has a thickness ranging from 10 to 400 microns. The substrate and the first selective emitter material are selected so that thermal energy incident to the substrate is in turn emitted such that about 98% or greater of the power emitted from the thermally sprayed layer is in a range of infrared wavelengths greater than 800 nm.
In a second aspect, the present invention is directed to a composite emitter comprising a substrate having a first surface and a second surface laterally spaced apart from the first surface. A reflective metal layer contacts the first surface of the substrate. A thermally sprayed layer contacts one of the second surfaces of the substrate and the reflective metal layer. The thermally sprayed layer comprises a selective emitter material and has a thickness ranging from 10 to 400 microns.
In a third aspect, the present invention is directed to a composite emitter for selectively emitting energy resulting from thermal energy incident to the composite emitter. The composite emitter comprises a substrate having a first surface and a second surface laterally spaced apart from the first surface. A thermally sprayed layer confronts one of the first and second surfaces and has a thickness of about 10 microns to about 400 microns. The thermally sprayed layer consists of a substantially pure ceramic oxide selective emitter material.
In a fourth aspect, the present invention is directed to a method of forming a composite emitter for selectively emitting energy resulting from thermal energy incident to the composite emitter. First, a substrate is provided. The substrate is made of a first material and has a first surface and a second surface laterally spaced apart from the first surface. Next, a layer of a substantially pure ceramic oxide selective emitter material is thermally sprayed onto the substrate such that the layer has a thickness ranging from about 10 microns to about 400 microns.
In a fifth aspect, the present invention is directed to a method of fabricating a composite emitter. First, a substrate is provided. The substrate is made of a first material and has a first surface and a second surface laterally spaced from the first surface. Next, a layer of a selective emitter material is deposited by thermal spraying such that the layer has a thickness ranging from about 10 microns to about 400 microns. The first material and the selective emitter material are selected so that thermal energy incident to the substrate is in turn emitted such that about 98% or greater of the power emitted from the layer of selective emitter material is in range of infrared wavelengths greater than 800 nm.